Receiver and diversity receiver system



July 4, 1950 l.. R. KAHN 2,513,803

RECEIVER AND DIVERSITY RECEIVER SYSTEM LEONARD WN BY/ l I ATT RN EY July 4, 1950 L. R. KAHN 2,513,803

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INVENTOR LEO ARD R. KAHN BY f--f ATTO R N EY Patented July 4, 1950 RECEIVER AND DIVERSITY RECEIVER SYSTEM Leonard R. Kahn, New York, N. Y., assigner to Radio Corporation of Delaware of America, a `corporation Application January 29, 1948, Serial No. 5,018

4 Claims. (Cl. Z50-8) In this application I disclose a receiver for signals with improved means to reduce noise in the receiver output resulting from fading 'of the 'signal being received.

The invention .is described as being used in telegraphy systems of the on-off type `and frequency shift type. In on-off telegraphy signals, a carrier is transmitted and as transmitted is interrupted,'being modulated to zero, with either the .cn o1' olf periods representing signal elements. For positive signal recorder or rebroadcast control, in practice the 'signal on period is usuallyr used .to represent signal elements, In frequency shift telegraphy, the carrier is on continuously but its frequency is shifted from one value to another value and back with one frequency value representing marking intervals, the other frequency value represent-ing spacing intervals.

In practice, as is well known, the signalcarrier may fade out so that the signal-to-noise 'ratio becomes unfavorable and this results in distorted signals which are referred to as garbled signals in telegraphy practice.

An object of the .present invention is to improve the signal-to-noise ratio -of la signalling system. In the description which follows, reference is made to telegraphy systems of the frequency shifted and on-off type but thev invention is applicable to voice modulation, facsimile, television and the like, in fact, to any Isystem wherein a limiter is used in the signal ychannel before detection takes place.

In telegraphy systems `as known today, diversity effects are made use of. 'Ihe diversity effects may vresult v'from physical spacing of two or more antennas picking up the same signal lor from use of different frequencies or from use of antennas with different polarization characteristics. These diversity effects are used to obtain the best signal 'and to overcome theeifects of fading as described above since fading at difterent points is different and -at different frequencies is different and fading `effects on antennas of different polarization are `also different.

A further object of my invention is tol inf-17 prove the vsignal-.to-noise ratio in the individual receivers of a diversity system and to also ,prof

appears. These systems 'are somewhat compli`` cated. Especially is this fso where more than' two channel diversity effects Aare to be used. Other systems Vmake use 'of 'a vcommon detector load wherein a potential is developed which builds up in such a manner as to block all of the Vdetectors `except that one being supplied the strongest signal. In these systems, the weaker channels are not 'completely blocked so' that the action is combining rather than selection 'of one' signal only. Moreove'nthese 'systems 'areinot applicable to frequency shift reception because a limiter isused infeach receiver' 'and the outputs are of the same magnitude and would be ineffective in a common load 'to block the weaker` channels. However, in these systems, 'the use of more than two channels in diversity is relatively simple and a further object of my invention is 'to improve the switching action in a diversity system of the latter type and in particular in frecguencyl 'shift' keying systems `comprisingthree'or more channels in diversity. l

' In on-orf telegraphy systems, noise in the receiver in thesig'naloif or "space` conditie-n. may operate the'recording apparatus andafurther 'object 'of my invention is to provide means to block all loutput from the receiver in the intervals between marking elements. In describing my'invention in detail, reference will-'be made to the attached drawings' wherein- Fi'gs. ',1 and 2 illustrate two rembodiments-of diversity systems arranged and operated in accordance with my invention. In Fig. 1, the receiver is larranged to receive frequency modulated sign'als.y vIn Fig. 2 the apparatus is vconditioned to receive amplitude modulated signals ofany typewhich .may be limited before detection. I

Fig; -3 illustrates an on-off telegraphy receiver making use of my invention, While,

Fig. 4 illustrates la blocking detector system tem of which two .channels areshown. Therey tube in the Channel Whereinthe strongest signal 55 may be more channels addedas desired. 4.The two channels are designated channels A and B. In'v describing the, system, reference .will be made tothe elements of channel A since to some extent,- the channel-s, are similar., Where similar elements are found in channel B, corresponding'.

references primed will be applied thereto.

channel A comprises an antenna system located at a spaced point or .in different polarization v,with respect t.0,th9 antennasystem |07-,

Thefantenna Sys-tem A feedsone. version Off- .the signal.v to, apparatus in unit |2 apparatus receiver antenna system I0 or I0'.

mediate frequency energy which carries the signal is supplied to an amplifier I8 andA from the amplifier I8 to a limiter 20." The amplifier IB isagain of fixed adjustable gain so that its output magnitude is proportional to the; magnitude: of energy picked up at l0. The limiter in 20 supplies its output to a band pass filter 22, the purpose of which will be described hereinafter, and from this band pass filter 22 to a detector unit 24, This unit 24 in the embodiment being described isa discriminator and detector such as, for example. shown in Conrad Patent #2,151,747, dated March 28, 1939, and Seeley U. S. Patent #2,121,103, dated June 21, 1938, or modifications thereof. The discriminators each include This intera'detector and the outputare filtered and may ate a tone keyer feeding a recorder or teletype-v writer or it may feed the teletypewriter or recorder more directly.

Intermediatefrequency output is also supplied by lead I5 to aseparate rectifier circuit'in 3E.V Intermediate frequency outputfrom the other channels is also'supplied to separate rectiers and the rectiers have a common load imped--U ance so that rectified current appears in said i common load. The diodeA rectifiers are so arranged that thev one supplied with the weaker signals are blocked by the potential developed in the common load by the rectiiied signal from the channel getting the strongest signal. The reci tied signals therefore develop across the common load a potential the magnitude of which is proportional to the strength of the signal in the channel getting the strongest signal. The principle involved here isin general as described in A satisfac- Peterson U. S. Patent #2,290,992. tory blocking detector system is shown in Fig. 4.

Transformers 'I'. T and T"fee'd the IF current to rectiers D, D' land D connected in like polarity to a common load 31. The potential at the end of resistor 31'adjacent the diode cathodes is positive when current is rectified and the loadyisso proportioned that this positive poi tential on thejdiode Icathodes with `respect tov the negative' potential'vonythe diode anodesj,A isv suicient to block" the "diodes except that one supplied with the' strongest' signal.

65 The potentialldrcpin theA loadV impedance 3'!y is used to controlt thegain 4of @variable gain l amplifier 38 supplying oscillations from unit' 40 by way of ar'n'pliiierl 42 tothe limiter 2,0 input.

` The oscillator 40 referred to herein as a localV comparison frequencywoscillator since the oscillations are supplied to the limiter 20 and in a sense compared as to magnitude with the signal supplied thereto -from: the,.amplifiers, I 8. The

purpose of this is to make `use of the thresholding characteristic of a limiter to pass therethrough signal from I8 to the exclusion of noise and oscillations from 40 or if the signal -is too weak, oscillations from 40 to the exclusion of noise and the signal.' The amplifiers I8, I8 and Y 42 and 42 have additional functions of isolating theireceiversIZl and l2 fromV each other and from-their limiters.

I do not want to supply oscillations from 40 to the discrimator and detector 24, in the event they are of magnitude greater than the signal and noise, and pass through the limiter. I, therefore, provide the band pass filter at 22 and make the frequency of the oscillations 40 suiiiciently diiferentfrom the frequency of the intermediate frequency energy supplied to the limiter and filterA 22 to be blocked by lter 22. In other words, only the intermediate frequency including the carrier and necessary side bands passes through the lter 22. l

In operation, the amplitude of the oscillations supplied by the variable gain amplifier 38 from oscillator 40 to limiter 20 is automatically maintained, by amplifier gain andadjustment of the rectifier circuits in 36, a few decibels below the level of the Vsignal from the strongest channel at its limiter input. This oscillation magnitude is determined by operationiof the receiver andits antenna location and amplifier gain and adjust# ment is made to obtainA the'values. as desired.l Each limiter receives the output of its receiver I2 and also oscillations from oscillator 40. If the receiver output is'of lesser' amplitude thanthe local comparison frequency oscillations from 40,

the said oscillations will suppress that receiver output. The strongest receiver output will pass through its limiter and will determine the diver-- sity units output. The other limiters will supply oscillations from the low frequency comparison source 40 which will not pass through the band pass filter.

The limiter in- 20 may be conventional. It is essential, however, that the limiter be sensitive to input voltage differences so that the threshv olding actionis complete and the output representsthe strongest input signal. A limiteras disclosed in Crosby U. S. Patent #2,276,565 is satisfactory for use if the intermediate frequency is not too high.

In 'the embodiment of Fig. 2, which is for on-off, keying, three channels are assumed. More channels may be used. The antenna I0, receiver I2, limiter 20 and lter 22 with thevariable gain amplifier 38 may be substantially as illustrated and described above. The detector 24 is of the amplitude modulation typesince I have assumed the signals here to comprise carrier and sideI band energy keyedA on and off in accordance with telegraphy signals. The apparatus in 3B may also be as described above and may comprise a common load 31 fed by rectified current from a separate rectier coupled to the lines I6, I6 etc. Here, however, a capacitor '50 is connected across the common load impedance 3l so that the gain control potentials dok not follow the keying. Here again oscillationsof .controlled amplitude are supplied to the limiter 20. For example, these oscillations'may be of 50.3 kc. assuming that the intermediate'freque'ncy out of the receiver I2 is 50 kc. ithe necessary side bands Which-may, for example, comprise 250 cycles on each side. Then the filter 22 will pass only the signal and will block oif thelocal space frequency oscillations from runit '40 in the event theyget through the limiter by virtue -ofthresl'iolding action. The amplitude of the local space frequency oscillations from unit 49 is controlled by the potential developed across common load 31 which is in a network with a long time constant so that it does not vary with the incoming signal keying but is proportional to the average signal strength. The magnitude of the oscillations is adjusted to be a few decibels below the magnitude of the 50 kc. output of the receiver having the strongest signal. The incoming on- 01T signal, the noise and the said oscillations are fed to the limiter 20. When the on-o signal is on the limiter in the strongest channel suppresses the local space frequency oscillations and the noise. Then the signal only is fed to the filter 22 and detector 24. When the signal is olf the local space frequency oscillator suppresses the noise. Oscillations from 40 cannot pass through the filter 22 and the output of the unit is Zero. The remaining channels may be arranged in like manner and all of the detectors may supply output to a common load CL.

My invention is applicable to single receivers of the on-off telegraphy type, to reduce noise on space or off condition. Then on space the oscillations from 40 take over and due to the thresholding action in limiter 2l! block all noise and signal from the output. The output across the load CL is always constant on mark due to the limiting action when the signal takes control of the limiter. Such an arrangement is shown in Fig. 3, where a single receiver is used. The receiver AVC potential -rnay be fed to the variable gain amplified to control the magnitude of the oscillations from source 49 fed to the limiter. It is to be kept in mind that the magnitude of the signal input to the limiter is to be properly related to the magnitude of the oscillations from 40. Here both are made -proportional to the signal picked up in the signal on ,condition.

What is claimed is:

1. In a diversity system, in combination, a plurality of signal receivers each comprising a limiter, a lter and a detector in cascade in the order given, said lters each being adjusted to pass a band of frequencies including the signals, a common output circuit coupled to all of said detectors, a source 0f oscillatory energy of a freguency outside the pass band of said lter, a variable gain amplier coupling said source to the input of the limiter in each receiver, a rectifier for each receiver excited by the signal therein, a common load impedance for said rectiiers, and means for controlling the gain of said amplifier in accordance with the potential across said common load impedance.

2. In a diversity receiver, in combination, a plurality of signal pick-up devices responsive to radiant energy signals, a common output circuit, a plurality of signal amplifiers each including a limiter and a discriminator detector, there being a signal amplifier coupled to each pick-up device, a. circuit coupling the outputs of all of the discriminator detectors to said common output circuit, a source of oscillations, connections between said source and the inputs of all of the limiters, a variable gain amplifier in a common portion of said connections, a rectifier coupled to each of said rst named amplifiers, a common load for said rectiers, and means for controlling the gain of said variable gain amplifier in accordance with the potential developed in said common load.

3. In a diversity receiver for on-off telegraphy signals, in combination, a plurality of signal pick-up devices responsive to said signals, a common output circuit, a plurality of signal amplifiers each including a limiter and a detector, there being a signal amplifier coupled to each pick-up device, a circuit coupling the outputs of all of the detectors to said common output circuit, a source of oscillations, connections between said source and the inputs of all of the limiters, a variable gain amplifier in a common portion of said connections, a rectifier for each pick-up device excited by energy proportional to the signal picked up thereby, a resistor and a capacitor in a time delay network connected to all of said rectiers, and connections from said resistor to said variable gain amplifier for controlling its gain in accordance with the current in the common output of said rectiiiers.

4. In a diversity system, a plurality of signal receivers each including a signal amplifier, all of which ampliers are of like over-all gain, a detector for each signal receiver, means coupling each detector to the output of its corresponding amplifier, a current amplitude limiter in each coupling for feeding the signals to the corresponding detector, a source of oscillations of a frequency different from the signal frequency, connections between said source and the inputs of all of said limiters, a variable gain amplifier in said connections, apparatus for controlling the gain of the last named amplier in accordance with the magnitude of the signals supplied to the rst named amplifiers, and a common output coupled to all of the detectors.

LEONARD R. KAI-IN.

REFERENCES CITED The following references are of record in the o le of this patent:

UNITED STATES PATENTS Number Name Date 1,373,931 Alexanderson Apr. 5, 1921 2,059,321 Drake Nov. 3, 1936 2,087,283 Landon July 20, 1937 2,108,117 Gardere et al. Feb. 15, 1938 2,212,540 Haffcke Aug. 27, 1940 2,214,929 Koschmieder Sept. 17, 1940 2,253,867 Peterson Aug. 26, 1941 2,316,851 Foster Apr. 20, 1943 2,387,632 Young Oct, 23, 1945 2,395,737 Hansell Feb. 26, 1946 2,451,916 Chapin Oct. 19, 1948 2,462,224 Rheams Feb. 22, 1949 

